This invention was made under contract with or supported by the Electric Power Research Institute, Inc. of Palo Alto, Calif.
1. Field of the Invention
With the increasing scarcity of oil and gas in conjunction with the continuously increasing price, the relative abundance of coal in the United States has greatly encouraged investigation to expand the usefulness of coal. Coal has had limited application because of high heteroatom content, particularly sulfur, and because it is a solid. Therefore, processing has been directed toward reducing the sulfur content, while enhancing the ability to melt a coal extract, which is substantially freed of inorganic materials. In addition, the processing has involved the introduction of hydrogen into the coal extract, so as to enhance its heating value and reduce the sooty character when it is burned.
A common process for coal liquefaction is referred to as solvent refined coal (SRC). In this process, coal, normally finely comminuted, is contacted with a hydrogen donor solvent at elevated temperatures, preferably in the presence of hydrogen, optionally in the presence of a catalyst. The resulting product is freed of inorganic materials and then separated into various fractions. In this process, there are many variables which enhance or diminish the economic attractiveness of the process. One consideration is the efficiency of hydrogen introduction into the coal fraction of interest, as compared to hydrogen useage for volatile hydrocarbons and the formation of water. A second consideration is the efficiency with which heteroatoms are removed, particularly sulfur and oxygen. A third consideration is the nature and yield of the desired product. A further consideration is the conversion of the coal into useable products.
In developing a process, the above as well as other considerations must be taken into account. Because of the extraordinarily large volumes of materials which are involved, small differences in efficiency or yield are of substantial significance. In any process which is developed, it is desirable that the materials employed in the process are for the most part generated internally.
2. Description of the Prior Art
Copending application Ser. No. 733,832, filed Oct. 18, 1976 describes a two-stage process of which the subject invention is an improvement. The relevant portions of that application, which describe the two-stage process in detail, are incorporated herein by reference.